service manual for a WESCO heat pump...model SP36-1

[certified tech group 51]

 Model number SP36-1 serial number 1665..   Customer states the part outside ( fan and coils )  gets frost and ice on the unit when it is cold outside, when he has the unit on calling for heat inside..(there is a rotary switch with numbers  inside the cabinet with the outside coils, T-stat?) ....have not worked on too many heat pumps.... ( It is only a refrigeration unit with a reversing coil and fancy controls, how hard can it be, 240 volts here, 24 volts over there, large whirling blade )

[applianceman18007260692]

More than likely a defrost issue. The unit has to defrost every so often or ice will build on the outside unit. Their is a defrost board mounted on the unit somewhere. I would start there.

[dkpd1581]

The box and knob is a Ranco brand E-15 Time-Temperature Initiation and Temperature Termination Control defrost module. 

The heat pump will frost in the heating mode and this is normal; however, if it becomes sheet ice and freezes throughout, you have a defrost control issue.

One of the best books to learn about Heat Pumps is by Richard Jazwin The HVAC/R Professional's Field Guide to Heat Pump Systems & Service (page 54 has the Ranco E15 in it if you want some info).  There are 2 versions of the book the larger on has LOTS of OEM's in there for older units which comes in real handy in the field and to learn from.  The smaller book is the exact same except for the OEM info.

Best of luck.